I. Field of the Invention
This invention relates to underreamers and expansible cutters and, in particular, to an underreamer drill bit with a conical cutter having an improved cutter mounting.
II. Description of the Prior Art
Underreamers, expansible cutters and hole openers have been used for enlarging well bores either connected to a drill string above the drill bit or in the string without any drill bit on the lower end of the string. U.S. Pat. No. 3,386,521 is an example of the combined drill bit and underreamer and U.S. Pat. No. 3,171,503 is an example of an underreamer on the lower end of the drill string. U.S. Pat. No. 4,533,003 illustrates an improved means for securing a cutter to an underreamer arm.
Conical cutters have been used in drill bits and underreamers. Such conical cutters have been installed on a pin which extends downwardly and inwardly from the lower end of the underreamer arms. Roller bearings and balls have been used in mounting the conical cutters on the arms as shown in U.S. Pat. No. 2,641,447. Pins or balls locked in a groove between the arm and the interior of the conical cutter have been used to secure the cutters on their pins as shown in U.S. Pat. Nos. 2,661,447; 3,998,500; and 2,519,716. U.S. Pat. No. 4,161,343 discloses the use of a friction bearing and a thrust button between the pins and the cutters. Pat. No. 3,998,500 suggests the use of a bolt to secure a bearing sleeve on the pin. U.S. Pat. No. 4,478,299 shows a variety of bearings on the pin including a split bushing arrangement.
The amount of cutting that can be obtained from a conical cutter on an underreamer has been limited by the time in cutting service during which the cutter rotates relatively freely and this is a function of the efficiency of the mounting.
In the conventional conical cutter, i.e. cone bit, the cone is designed to roll around a circle as the cone bit turns. The radius of this circle is referred to as the natural rolling radius of the cone. As long as this rolling radius remains constant, wear on the cutter and its mounting assembly is relatively even and the cone bit will experience a good drilling life. However, when a cone bit is used in an underreamer, as the underreamer arms expand outwards in operation, the rolling radius is increased beyond the cone's normal rolling radius. The normal thrust load in a cone bit is directed head on against the cutter element. However, as the arms of the underreamer open, essentially all the loading on the cutter assembly is a reverse thrust load applied against the side of the cutter element. This reverse thrust load is the major element of the loading on the cutter assembly in an underreamer, particularly at the beginning of operation or if the underreamer only cuts a small shelf, because the underreamer only uses the outer rear edge of the cutter cone. Further, in a highly deviated hole, the reverse thrust lead is predominate. As the cutter assembly opens during operation, the reverse thrust load reduces, but remains a significant element of the loading on the cutter assembly. Additionally, during operation the cone bit always tries to return to its natural rolling radius; i.e. to close the underreamer or track-in which creates its own load against the cutter assembly. These varying loads placed upon the cutter assembly during operation cause excessive wearing and uneven wearing of the cutter mounting assembly. One effect of such uneven wearing and excessive wearing is rapid deterioration of the pin on the underreamer arm and the bearings which form a part of the cutter assembly. In practice, when wear of the pin on the underreamer arm occurs, the end portion of the underreamer arm is cut off and a new portion is welded on. The heat of such welding weakens the underreamer arm. Additionally, each time the arm is cut to weld on a new end portion, more of the original arm must be removed to replace the heat weakened section. Accordingly, the cutter assembly can be replaced only a few times.